linux-kernel - Re: [PATCH v2 2/3] x86/entry: Fix entry/exit mismatch on failed fast 32-bit syscalls

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Message-ID: <87h7luubqv.fsf@nanos.tec.linutronix.de>
Date:   Mon, 01 Mar 2021 16:36:40 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>, x86@...nel.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH v2 2/3] x86/entry: Fix entry/exit mismatch on failed fast 32-bit syscalls

On Tue, Feb 23 2021 at 10:15, Andy Lutomirski wrote:
> On a 32-bit fast syscall that fails to read its arguments from user
> memory, the kernel currently does syscall exit work but not
> syscall entry work.  This confuses audit and ptrace.  For example:
>
>     $ ./tools/testing/selftests/x86/syscall_arg_fault_32
>     ...
>     strace: pid 264258: entering, ptrace_syscall_info.op == 2
>     ...
>
> This is a minimal fix intended for ease of backporting.  A more
> complete cleanup is coming.
>
> Cc: stable@...r.kernel.org
> Fixes: 0b085e68f407 ("x86/entry: Consolidate 32/64 bit syscall entry")
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/entry/common.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 0904f5676e4d..cf4dcf346ca8 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -128,7 +128,8 @@ static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
>  		regs->ax = -EFAULT;
>  
>  		instrumentation_end();
> -		syscall_exit_to_user_mode(regs);
> +		local_irq_disable();
> +		exit_to_user_mode();

That's still the same as the previous version. The right function (while
the name is misleading) to invoke here is irqentry_exit_to_user_mode()
because that invokes exit_to_user_mode_prepare() before
exit_to_user_mode(). We can rename that function afterwards.

Thanks,

        tglx